The invention relates to a magnetic-tape apparatus for recording/reproducing signals on/from a magnetic tape which is movable between two reels in a first transport direction and in an opposite second transport direction, and includes a tension or tape-pull control device for controlling the longitudinal tension or tape pull in the part of the magnetic tape between the two reels. Such apparatus, whether used for both recording and reproducing, or only either one of the functions, will be referred to hereinafter, in the specification and claims, as a tape recorder.
The tension control device comprises a sensing lever which is mounted on the apparatus chassis so as to be pivotable about a pivoting axis in a first pivoting direction and in an opposite second pivoting direction a spring which acts on the sensing lever to urge it in the first pivoting direction; a tape-contact pin near the first end of the sensing lever, which when a tape is present is in contact with the part of the magnetic tape which extends between the reels; a braking disc on one of the two reel supports; and an elongate flexible braking member which surrounds at least a part of the circumference of the braking disc and which has a first end secured to an apparatus part different from the sensing lever, (that is, a part other than the lever). The braking member co-operates with the sensing lever to produce a tensile force in the braking member varying with the position of the sensing lever, and thereby influences the braking torque.
German Patent Specification No. 1,549,107 to which U.S. Pat. No. 3,580,525 corresponds, describes such a magnetic-tape apparatus in which the braking member is a brake band. In this known apparatus the tape pull for each of the two tape reels is controlled by means of an associated tension control device. Recording or reproducing of signals on the magnetic tape is effected only during transport of the magnetic tape in one of the two directions of transport. Each of the two tension control devices has a tension spring which pulls at the free second end of the relevant brake band. The sensing levers also cooperate with these free second ends to vary the braking torque.
It is advantageous if a relatively small displacement of the sensing lever results in a comparatively large variation of the braking torque. Moreover, it is desirable that the tape contact means of the sensing lever exert only a small force on the magnetic tape, so that small variations in the position of the magnetic tape and in the force exerted on the magnetic tape by the sensing lever result in large variations of the braking torque.
It is known that in the case of a flexible braking member such as a braking wire, a braking cord or a braking band which cooperates with a braking disc, the friction force exerted on the braking disc by the braking member increases exponentially in the direct of rotation of the braking disc. Therefore, it is advantageous if the friction force exerted on the braking member by the braking disc decreases towards the point where the brake lever cooperates with the braking member. In the known magnetic-tape apparatus this is the case in the "playback" mode.